Commit 858274b6 authored Jan 05, 2017 by Waiman Long Committed by Thomas Gleixner Feb 05, 2017

debugobjects: Reduce contention on the global pool_lock



On a large SMP system with many CPUs, the global pool_lock may become
a performance bottleneck as all the CPUs that need to allocate or
free debug objects have to take the lock. That can sometimes cause
soft lockups like:

 NMI watchdog: BUG: soft lockup - CPU#35 stuck for 22s! [rcuos/1:21]
 ...
 RIP: 0010:[<ffffffff817c216b>]  [<ffffffff817c216b>]
	_raw_spin_unlock_irqrestore+0x3b/0x60
 ...
 Call Trace:
  [<ffffffff813f40d1>] free_object+0x81/0xb0
  [<ffffffff813f4f33>] debug_check_no_obj_freed+0x193/0x220
  [<ffffffff81101a59>] ? trace_hardirqs_on_caller+0xf9/0x1c0
  [<ffffffff81284996>] ? file_free_rcu+0x36/0x60
  [<ffffffff81251712>] kmem_cache_free+0xd2/0x380
  [<ffffffff81284960>] ? fput+0x90/0x90
  [<ffffffff81284996>] file_free_rcu+0x36/0x60
  [<ffffffff81124c23>] rcu_nocb_kthread+0x1b3/0x550
  [<ffffffff81124b71>] ? rcu_nocb_kthread+0x101/0x550
  [<ffffffff81124a70>] ? sync_exp_work_done.constprop.63+0x50/0x50
  [<ffffffff810c59d1>] kthread+0x101/0x120
  [<ffffffff81101a59>] ? trace_hardirqs_on_caller+0xf9/0x1c0
  [<ffffffff817c2d32>] ret_from_fork+0x22/0x50

To reduce the amount of contention on the pool_lock, the actual
kmem_cache_free() of the debug objects will be delayed if the pool_lock
is busy. This will temporarily increase the amount of free objects
available at the free pool when the system is busy. As a result,
the number of kmem_cache allocation and freeing is reduced.

To further reduce the lock operations free debug objects in batches of
four.

Signed-off-by: Waiman Long <longman@redhat.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: "Du Changbin" <changbin.du@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jan Stancek <jstancek@redhat.com>
Link: http://lkml.kernel.org/r/1483647425-4135-4-git-send-email-longman@redhat.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

parent 97dd552e

lib/debugobjects.c

+23 −9

Original line number	Diff line number	Diff line
		@@ -172,25 +172,39 @@ alloc_object(void addr, struct debug_bucket b, struct debug_obj_descr *descr)

		/*
		* workqueue function to free objects.
		*
		* To reduce contention on the global pool_lock, the actual freeing of
		* debug objects will be delayed if the pool_lock is busy. We also free
		* the objects in a batch of 4 for each lock/unlock cycle.
		*/
		#define ODEBUG_FREE_BATCH 4

		static void free_obj_work(struct work_struct *work)
		{
		struct debug_obj *obj;
		struct debug_obj *objs[ODEBUG_FREE_BATCH];
		unsigned long flags;
		int i;

		raw_spin_lock_irqsave(&pool_lock, flags);
		while (obj_pool_free > debug_objects_pool_size) {
		obj = hlist_entry(obj_pool.first, typeof(*obj), node);
		hlist_del(&obj->node);
		obj_pool_free--;
		debug_objects_freed++;
		if (!raw_spin_trylock_irqsave(&pool_lock, flags))
		return;
		while (obj_pool_free >= debug_objects_pool_size + ODEBUG_FREE_BATCH) {
		for (i = 0; i < ODEBUG_FREE_BATCH; i++) {
		objs[i] = hlist_entry(obj_pool.first,
		typeof(*objs[0]), node);
		hlist_del(&objs[i]->node);
		}

		obj_pool_free -= ODEBUG_FREE_BATCH;
		debug_objects_freed += ODEBUG_FREE_BATCH;
		/*
		* We release pool_lock across kmem_cache_free() to
		* avoid contention on pool_lock.
		*/
		raw_spin_unlock_irqrestore(&pool_lock, flags);
		kmem_cache_free(obj_cache, obj);
		raw_spin_lock_irqsave(&pool_lock, flags);
		for (i = 0; i < ODEBUG_FREE_BATCH; i++)
		kmem_cache_free(obj_cache, objs[i]);
		if (!raw_spin_trylock_irqsave(&pool_lock, flags))
		return;
		}
		raw_spin_unlock_irqrestore(&pool_lock, flags);
		}

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